I've found that with supporters (i.e. jock straps) you want to get a size larger than the advertised size; there's a lot of elastic in those things, and wearing the "correct" size can be very uncomfortable. As for the cup itself, I have no experience with metal.

Hey guys... maybe the wrong place to ask but can you recommend a good, comfortable metal groin guard please?

Also how do the sizes work? Is it the size of my waist or the size of my junk? lol

Cheers,

It's the size of your waist. All of the cups are the same size except for the Child/Adult distinction.

I'm afraid we'll need one of the Muay Thai folks to weigh in on a metal cup for you. I use a flex cup that is rubber and definitely not suited to striking. Two of the MT guys that train at our gym wear these: http://www.cagesidemma.com/camuthstcup.html but I totally couldn't give you an evaluation of them compared to other metal cups.